Fellow endlessly challenging high ranking official (8)
I believe the answer is:
mandarin
'high ranking official' is the definition.
(mandarin can mean a powerful bureaucrat)
'fellow endlessly challenging' is the wordplay.
'fellow' becomes 'man' (fellow can mean a man).
'endlessly' means to remove the last letter.
'challenging' becomes 'daring' (daring is a kind of challenging).
'daring' with its final letter taken off is 'darin'.
'man'+'darin'='MANDARIN'
(Other definitions for mandarin that I've seen before include "Bigwig" , "Top civil servant" , "High official in imperial China and a fruit" , "Drain man (anag) - high-ranking Chinese official" , "High-ranking official or small orange" .)